docs: speculation.txt: mark example blocks as such

Identify the example blocks there, in order to avoid Sphinx
warnings.
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/speculation.txt

@@ -17,7 +17,7 @@ observed to extract secret information.
 
 For example, in the presence of branch prediction, it is possible for bounds
 checks to be ignored by code which is speculatively executed. Consider the
-following code:
+following code::
 
 	int load_array(int *array, unsigned int index)
 	{
@@ -27,7 +27,7 @@ following code:
 			return array[index];
 	}
 
-Which, on arm64, may be compiled to an assembly sequence such as:
+Which, on arm64, may be compiled to an assembly sequence such as::
 
 	CMP	<index>, #MAX_ARRAY_ELEMS
 	B.LT	less
@@ -44,7 +44,7 @@ microarchitectural state which can be subsequently measured.
 
 More complex sequences involving multiple dependent memory accesses may
 result in sensitive information being leaked. Consider the following
-code, building on the prior example:
+code, building on the prior example::
 
 	int load_dependent_arrays(int *arr1, int *arr2, int index)
 	{
@@ -77,7 +77,7 @@ A call to array_index_nospec(index, size) returns a sanitized index
 value that is bounded to [0, size) even under cpu speculation
 conditions.
 
-This can be used to protect the earlier load_array() example:
+This can be used to protect the earlier load_array() example::
 
 	int load_array(int *array, unsigned int index)
 	{